Blue Ivy Is Helping Mom Beyoncé with New Twins: 'She's the Best Big Sister,' Says Source

Add being a dutiful big sister to Blue Ivy's budding resume, which already includes being a rapper and future style icon. A source tells PEOPLE that the 5-year-old has been doting on her newborn siblings -- twins Rumi and Sir -- and even assists mom Beyoncé and dad JAY-Z with daily baby duties. "Blue is the best big sister," says the source. " helps Beyoncé every day. "Everyone is doing great," adds the source. "They are still enjoying the summer in Malibu. Blue is excited to be back in school...